THE OSTEOLOGY OF ELOTHERIUM. i 281 
in Sus. The superior border curves. upward into a great, hook-shaped process, which 
resembles that seen in Merycochawrus, and gives a highly characteristic appearance to this 
region of the skull. That portion of the zygomatic process which is directed anteriorly 
is short and, though massive, is much less so than that which extends out laterally ; in 
front it is received into a notch of the jugal. The glenoid cavity is large, transversely 
directed and quite deeply concave, though the postglenoid process is not strongly deyel- 
oped and is hardly more conspicuous than the preglenoid ridge. This disposition is 
unusual among the ungulates, but it occurs also in the Eocene genus Achanodon and in 
the modern Dicotyles. The glenoid cavities of the two sides are very widely separated, 
their inner margins lying external to the line of the paroccipital processes. The posttym- 
panic process of the squamosal is small, and is closely applied to the paroccipital process. 
The shape of the zygomatic arches, together with the extreme narrowness of the cranium 
proper, causes the temporal openings to be very large and to appear widely open when 
the skull is viewed from above. ‘These openings are, however, less extended transversely 
and more antero-posteriorly than in Hippopotamus, while in Sus they are hardly visible 
from above. 
The jugal isa very remarkable bone and constitutes one of the most extraordinary 
features of the Hlotheriwm skull. Posteriorly it is notched to receive the zygoma, and 
sends out a process along the ventral face of that bone, extending to the preglenoid ridge. 
The jugal forms the inferior half of the nearly circular orbit, and for this purpose its 
dorsal border is made deeply concave, giving off a stout postorbital process to meet that 
of the frontal, while anteriorly it is moderately expanded upon the face in front of the 
orbit, where it is wedged in between the lachrymal and the maxillary. The most pecu- 
har feature of the jugal, however, is the immensely developed vertical plate, which 
descends from beneath the orbit downward and outward to below the level of the ven- 
tral border of the mandible, recalling the similar, but much Jess massive processes found 
in certain edentates, e. g., Megatherium. ‘These plates are laterally compressed, but quite 
thick, and when the skull is viewed from the front, they are seen to diverge quite 
strongly downward ; their shape varies in the different species. In the very large forms 
from the Protoceras beds, such as #. imperator, the process retains its plate-like form 
throughout, its free end being only moderately thickened. This appears to be true also 
of £. mortoni, though my material is not sufficient to allow me to make this statement 
positively, but in the large species from the Titanotherium and Oreodon beds (£. ingens) 
it forms a club-like thickening at the tip, which in /. ingens is coarsely crenulate on the 
posterior border (see Pl. XVII). These processes are, so far as is yet known, quite unique 
among the hoofed mammals, and it is difficult to form eyen a conjecture as to what their 
functional significance may have been. Some misunderstanding has arisen as to the spe- 
